Grinding and polishing operations are carried out in the conduct of various commercial, industrial and scientific pursuits. Some machinery for performing such operations is spherically tooled for lapping and polishing spherical surfaces. In some cases, the lapping tool is driven while in other cases the workpiece is driven against a stationary lap. In connection with such operations, while spinning the workpiece or lap, a randomizing of the grinding and polishing action is provided in order to distribute wear evenly over the whole surface of the workpiece. This has been accomplished in the past through causing the workpiece or the lap to be orbited or oscillated with respect to the spin axis of the tool. Such an action is called a "Breakup", especially when the extremities of the motion produced lie at different points. This functions to deter the creation of aberrations especially in optical flats, spherical or toric surfaces.
Surfacers of the high speed type to which this invention may be applied, have only recently been brought into broad usage within the ophthalmic lens trade. A good example of such surfacers are manufactured and sold as models numbered 504, 505, 506, etc. by Coburn Manufacturing of Muskogee, Okla. The 504, etc. series of machines was developed in answer to the challenge of severe limitations in the speed capabilities of the formerly available surfacing equipment, particularly those limitations in the performance characteristics of toric polishers which are also known as cylinder machines.
The earlier equipment incorporated various mechanisms for producing single axis oscillatory stroking action for surfacing a lens. Such machines had some features in common, characterized by the use of a work holder adapted to hold a lens rigidly in place while it was being surfaced. The driving mechanism was applied to driving the surfacing lap in relation to the surface of a fixed lens. A constantly changing randomized path of travel was produced while maintaining an axis of the lap firmly and constantly in parallelism with an axis of the lens. The net resulting motion was the product and combination of oscillatory drives directed along single axes arranged at ninety degrees from each other. The evolutionary improvements in such apparatus consisted principally in adding speed of drive to the single axis oscillatory motions. These increases in speed were found to produce heightened vibration and caused excessive wear and tear on the linkage. This equipment was subject to practical limitations by reason of the excessive vibration which developed as one increased the speeds at which such mechanical structures were attempted to be driven.
The economic conditions of competition in the lens making industry, with the attendant increases in labor cost, demanded that machines be produced having greater speed of relative lens and lap movement needed to improve surfacing capability. It became evident that some new and unique means for producing greater relative motion between lens and lap, per unit of time, was needed. This requirement posed severe problems. Accelerated relative motion of the oscillatory type in such equipment had to be provided with the axis of the lens curves always maintained in parallelism with an axis of the surfacing tool and lens curves. The speed limitations were first measurably overcome by advent of the Coburn 504, etc. series of machinery. In this construction, the surfacing tool was driven orbitally in a rotary motion with the surfacing tool maintained in constant axis alignment with an axis of the lens. A gimbal arrangement was utilized, through which a shaft extended and on which the surfacing tool was supported. The gimbal was fixed in a block which was journaled to allow rotation in the gimbal on pivots set ninety degrees apart for universal lateral movement, while being held fixed against rotational movement. Connected at the lower end of the tool supported shaft, was an eccentric drive unit which was arranged to drive the lower end of the support shaft orbitally. The motion of the tool at the upper end of the support shaft in the gimbal thus tracked exactly opposite to the drive action at the lower end of the shaft. In this way the tool on the upper end of the support shaft was driven in a circular orbit while the shaft remained fixed against axial motion. By this drive means a significant and meaningful improvement in speed of surfacing was achieved.
This equipment, while being an advance over prior designs, suffers from a high rate of mechanical attrition due to excessive wear. The wear thus occasioned within a short time affects the quality of lenses produced and the speed of surfacing.
The principal advantage of this 504, etc. series machinery is in the increased speed of surfacing achieved by driving the tool shaft orbitally. This series of machinery thus driven received immediate acceptance by the trade and at the present time is the mainstay of the industry in ophthalmic optical prescription processing. The machines are used in spite of their serious shortcoming of excessive early wear and the fact that they require repeated rebuilding to keep them servicable.
This advent and acceptance of the Coburn 504, etc. series equipment in spite of their shortcomings has firmly established the use of such machines having orbitally driven tool holders in the trade.
Another example of a surfacing machine having an orbitally driven tool is one manufactured by Howard Strasbaugh Company of Van Nuys, Calif.
Both the Coburn and Strasbaugh surfacers are provided with a break-up motion which causes the tool holder to be orbited on a first circular orbit about a first center of rotation which center is then eccentrically rotationally displaced on a second center of rotation in a second orbit to attempt to minimize the effects of excessive retracking which could result in making aberrations in the surface of the lens.
The above prior art equipment is adaptable to surfacing of lenses having spherical as well as cylindrical surfaces, but is used predominantly in the surfacing of lenses having cylindrical surfaces. Their principal contribution to state of the art has been to increase cylindrical surface polishing speed capability.
As is detailed below, the method and apparatus of the present invention provides an improved system for polishing such lenses over the above prior art machines described by driving the lens orbitally but in constantly changing patterns not limited to simply rotary movements. The patterns generated by the method of the present invention do not constantly retrack circular paths as in the above described Coburn and Strasbaugh types. The motions produced in equipment of the present invention range from essentially straight line action tracking along the path of one axis of the lens and then the other, and in between travel along the axes transitioning through elliptical, modified elliptical and then a limited number of circular orbits in constantly changing patterns. The need for a second centered orbital drive for purpose of providing a break-up is thus eliminated.
Cylindrical (toric) ophthalmic lenses of the trade comprise variable powers of magnification or minification which differ in range in optical power gradually between the two dominant radii of curvature placed ninety degrees apart the radii of the base and cross curves. Such lenses are used to neutralize astigmatism in a patient's vision.
In practice, such a lens when it is to be surfaced is "blocked" or mounted so that it becomes attached to a holder. The holder is provided with at least a pair of recessed conical detents lying along a line in the surface opposite to the surface where the lens is attached. Care is taken in blocking the lens to reference the intended axis of one of the dominant curves so that it lies in the plane of the line passing through the center point of the conical detents. These center points then become the referencing points to which lens or curve axes are registered or indexed in processing the lens through successive steps toward completion. The dominant curves which lie ninety degrees apart are called "base curve" and "cross curve" with all interlying curves taken on any given axis therebetween called intermediate curves. The lens when completed and gaged optically is stated to have a base curve power and a cross curve power separated arcuately by the aforesaid ninety degree spacing.
The lens and holder are first placed within a curve generator which can be set to cut a wide range of combinations of base and cross curves on the surface of the lens to be worked. The desired curves are then pre-set into the controls of the curve generator in accordance with the desired cylindrical prescription to be prepared. The lens is then diamond ground in the generator, excess stock is removed, and the desired curve combination is established.
When leaving the generator both curves have been established to approximate trueness and a rather rough lens surface having approximately the radii of curvatures is provided. This surface must then be fine ground, i.e, "fined", and then polished.
The remaining steps in completing the lens to the desired optical characteristics are performed on the surfacing apparatus of the prior art type first above described and according to the description of the improved method and apparatus of the present invention which ensues.
The first such additional step is the fining process. In this step, using the Coburn 504, etc. type equipment, a lapping tool which has been previously formed to the desired combination of toric curves is mounted on the driven tool holder and the lens positioned in contact therewith. A pair of pointed guide pins which are suppported on an arm arranged to oscillate along the axis of the base curve are placed in the conical detents of the lens block under pressure. These then move the lens and block back and forth over the orbiting tool on a fixed axis line. The oscillating arm rocks allowing the lens to tilt in the direction of the dominant axis of the lens in order to maintain contact with the lap while the tool is orbiting. This oscillating is done to establish an additional randomizing motion which combines with the eccentrically rotating axis of the orbiting lower tool to provide further "break up" motion.
The inventors of the present invention recognized that the prior art Coburn 504, etc. type orbitally driven apparatus, while adequate, needed to be improved upon to eliminate shortcomings of that equipment which result from simple orbital movement. These improvements take into account the fact that different motions and physical structures were needed. The cylinder (toric) lens surface as stated previously is comprised of two dominant curves on separate axes with myriad intervening curves. This produces an unusual surface on which the high spots must be removed quickly and the surface matched and reached to mate intimately in every part uniformly by the lap as soon as possible in order to surface the lens in a minimum of time to a maximum of truth in the optics produced.
It was postulated by the inventors that an improvement on the motions produced by the prior art apparatus was needed to speed and improve the lens polishing process. It was further postulated that producing a lapping drive motion which would cause tracking of the lap motion for a part of the surfacing cycle which would extend substantially along the axis of each of the base curves would produce better truth in optics. Also that maximum randomization could be reached by causing the motions to occur in a constantly changing elliptical, modified elliptical, and circular motion when transitioning from the essentially straight line motion tracking along the base curve line axis to the essentially straight line motion along the cross curve axis. It was also considered that speed of the polishing would thus be enhanced.
Accordingly, it is an object of the present invention to provide a randomized orbitally driven lens surfacing method comprising a drive motion which at times traverses the lens surface in substantially linear tracks along the axis of the base and the cross curve, and moves when transitioning therebetween through generally elliptically-orbital and circular motions in a constantly shifting variety of patterns.
It is a further object of this invention to provide method for producing multi-form surfacing patterns according to the method of the present invention.
It is a further object of the present invention to improve generally upon methods for surfacing toric and spherical lenses.
In practice under the invention, patterns are provided of progressively changing configuration. A stylus is constrained to move along mutually perpendicular axes. Sets of displacement forces are generated and applied to the stylus. The sets of such forces vary with time in magnitudes and senses to compel stylus movement in such progressively changing patterns.
Preferred apparatus for implementing this practice includes a pair of links having first ends pivotally connected to a stylus plate. A separate locus for movement of each of the opposite link ends is defined, by pivotally connecting such ends to separate wheels at off-center positions thereof. As the wheels rotate, each generates a set of displacement forces which is applied to the link connected thereto. With the links each at the same radial spacing on their wheels and with the links in phase, e.g., both at the nine o'clock positions, revolution of the wheels will displace a stylus on the stylus plate in a circular pattern. As the wheels are progressively misphased, the apparatus generates differing set of displacment forces, giving rise to displacement of the stylus in the above-noted progressively changing patterns.
In accordance with the method of the invention, realized in one specific practice by the above described apparatus, one selects and stores first and second sets of positional coordinates, each set having a corresponding number of elements. A pattern generator, e.g., stylus, is supported for movement along first and second different axes and is displaced selectively in accordance with the positional coordinate sets.
In the specific realization of the method in the above described apparatus, the positional coordinate sets are respectively in the two circular loci of the wheels. If one looks to sets of eight in number, the coordinates are those found at the eight (forty-five degree) spaced locations in each circular locus. As the wheels rotate into forty-five degree phase difference, the first set values are combined with the forty-five degree lagging second set values. Thus, combinations are made of the first element in the leading set with the eighth element in the lagging set, the second element in the leading set with the first element in the lagging set, etc., as rotation ocurs at such forty-five degree angle. The phase difference is preferably increased at a very slow rate, whereby many revolutions may occur before the wheels return to an in-phase condition and pattern change may be effected quite gradually. The out-of-phase relation may be effected providing identical circular loci and rotating the wheels at different speeds or by providing respectively different loci and rotating the wheels at the same speed.
The foregoing and other objects and features of the method and apparatus of the invention will be further understood from the following detailed description of preferred embodiments thereof and from the drawings wherein like reference numerals and literals identify like parts throughout.